Among the detection performed by an existing current transformer in China, according to a national standard GB/T 1208-1997 “CURRENT TRANSFORMER”, only a steady-state error of the current transformer is detected, so as to draw a curve of error of 5% or 10%. An accurate limit of the current transformer should satisfy the following standard:
currentphase displacementmultiple-error undererror underunder ratedprimary current withaccuracyrated primaryprimary currentrated accurate limitclasscurrent (%)±′ (minute)±crad(%) 5P±1601.8510P±3——10
With the development of national economy and the investment to the power system by the country, the power system with a high voltage and a large power grid is developed, and the short circuit capacity of the power grid increases quickly. Consequently, at the beginning of a system fault, the influence of the transient characteristic of a current transformer to the relay protection device can not be ignored. Presently, a state grid company establishes related industrial standards actively, and requires local grid companies to test the transient characteristic of the current transformer.
A primary time constant of a power grid directly influences the analyzing result of the transient characteristic of the current transformer. The primary time constants Tp in the systems which are located at different sites are different greatly. The Tp of a power transmission line is 20 ms to 30 ms, and the Tp of a large generator-transformer is larger than 200 ms. Consequently, the accurate calculation of the primary time constant of the power grid at current transformer installation sites is an important premise and base of detecting the transient characteristic of the current transformer.
The influence of Tp to a protective current transformer mainly includes the following.
A transient area coefficient Ktd of the current transformer is an important parameter for reflecting an anti-saturation ability of the current transformer. For a single power cycle C-t-O:
      Ktd    =                                        ω            ⁢                                                  ⁢                          T              p                        ⁢                          T              s                                                          T              p                        -                          T              s                                      ⁢                  (                                    ⅇ                              -                                  t                                      T                    p                                                                        -                          ⅇ                              -                                  t                  Ts                                                              )                    +      1        ,
in which Ts is a secondary time constant of the current transformer.
For two power cycles C-t′-O-tfr-C-t″-O:
            K      td        =                            [                                                                      ω                  ⁢                                                                          ⁢                                      T                    p                                    ⁢                                      T                    s                                                                                        T                    p                                    -                                      T                    s                                                              ⁢                              (                                                      ⅇ                                          -                                                                        t                          ′                                                                          T                          p                                                                                                      -                                      ⅇ                                          -                                                                        t                          ′                                                Ts                                                                                            )                                      -                          sin              ⁢                                                          ⁢              ω              ⁢                                                          ⁢                              t                ′                                              ]                ×                  ⅇ                                    -                              (                                                      t                    fr                                    +                                      t                    ″                                                  )                                                    T              s                                          +                                    ω            ⁢                                                  ⁢                          T              p                        ⁢                          T              s                                                          T              p                        -                          T              s                                      ⁢                  (                                    ⅇ                              -                                                      t                    ″                                                        T                    p                                                                        -                          ⅇ                              -                                                      t                    ″                                    Ts                                                              )                    +      1        ,
in which t′ is a time of a first power cycle, t″ is a time of a second power cycle, and tfr is a time of no current between two power cycles.
There are different values of the transient area coefficient Ktd of the current transformer calculated from different Tp, the difference of which is large.
Taking a measured current transformer of lgso20 type as example:
if Tp=100 ms, Ktd=10.1;
if Tp=60 ms, Ktd=8.18; and
if Tp=20 ms, Ktd=4.71.
It can be seen that the accurate calculation of the primary time constant Tp is an import premise of analyzing the transient characteristic of the current transformer.
In addition, the primary time constant Tp of the power grid determines an attenuation speed of a non-periodic component of the short circuit current. When a fault takes place outside of a line differential protection area, the current transformers at the two sides can detect the difference between the transient attenuation current. When the difference between the transient attenuation current exceeds a preset value of differential protection, a misoperation of differential protection will be caused.